A conventional optical wavelength tunable filter comprises a prism or a grating for selecting a predetermined wavelength of light. In the filter using the prism, for instance, the prism is rotated dependent on a wavelength of light to be tuned.
However, the adjustment of light axis is difficult in the conventional optical wavelength tunable filter. As a result, the efficiency is low. Further, the prism is not high in resolution power. This leads to the difficulty of providing a sharply defined spectrum. Still further, the filter can not be small in size due to the configuration of the prism. The same disadvantages are found in the filter using the grating.
In order to overcome these disadvantages, an optical wavelength tunable filter using an interference filter has been proposed as described in the Japanese Patent Kokai No. 62-22034. In this optical wavelength tunable filter, the inference filter is fabricated as multilayered dielectric thin films. The interference film filter has a good spectrum property, and a transmission wavelength is tuned by varying a light incident angle thereto. In this optical wavelength tunable filter, an optics is of a light collimated type, so that there are advantages in that the light axis adjustment is easy to be carried out, the cost is low, the size is small, and the efficiency is high.
Such an optical wavelength tunable filter is used in the field of optical communication, as described before. In an optical communication system, for instance, the above described interference filter is used to select one wavelength division from a wavelength division multiplexed signal light or to exclude spontaneous emission light as noise from a signal light amplified by an optical amplifier.
In such an application, there are the demands or needs to electrically tune a transmission wavelength. In case of a wavelength division multiplexed communication system, for instance, it is necessary to change a transmission wavelength, and to lock an optical wavelength tunable filter, through which a predetermined wavelength is transmitted, so that a receiving channel is selected. On the other hand, in case where the wavelength of a transmission light is fluctuated largely as compared to a transmission bandwidth of an optical filter, it is necessary that a feedback signal is supplied to the optical filter to control the transmission wavelength therethrough.
For this purpose, an optical wavelength tunable filter in which an interference filter is varied in angle by a pulse motor has been proposed.
In the optical wavelength tunable filter using the pulse motor, however, there are disadvantages set out below.
(1) It is difficult to be made small in size. PA1 (2) The continuous angle adjustment is not carried out due to the non-continuous rotation of a pulse motor, so that the fine adjustment of a transmission wavelength is not realized. PA1 (3) The control mechanism is complicated, and it is not suitable to be applied to a servo-system. PA1 an optical filter for transmitting a light of a predetermined wavelength band dependent on an incident angle of a collimated light propagated through a light path, the optical filter being rotatively positioned in the light path; and PA1 a piezoelectric actuator for generating a displacement dependent on an applied voltage; PA1 means for converting the diplacement to a rotation of the optical filter.